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NSnotificationcenter

对象之间进行通信最基本的方式就是消息传递,在Cocoa中提供Notification Center机制来完成这一任务。其主要作用就是负责在任意两个对象之间进行通信。使用方法很简单,如下几个步骤即可:

假设A与B之间进行通信,B来触发事件,A接受该事件,并作出响应。
1) A编写自定义的消息响应函数update
2) A向消息中心注册,[NSNotificationCenter defaultCenter] addObserver: self selector:@selector(update) name:@"update" object:nil]
3) B触发事件[[NSNotificationCenter defaultCenter] postNotificationName:@"update" object:nil]

每一个进程都有一个默认的NSNotificationCenter,可以通过类方法defaultCenter获取该消息中心的实例。消息中心可以处理同一进程中不同对象之间的消息。如果要在同一台机器上进行进程间的通信,需要使用NSDistributedNotificationCenter。

消息中心以同步的方式将消息分发到所有的观察者中,换言之,直到所有的观察者都收到消息并处理完毕以后,控制权才会回到调用者的手里。如果需要异步的处理消息,需要使用通知队列NSNotificationQueue。

在多线程程序中,通知会被分发到每一个发起消息的线程中,这可能与观察者注册时所在的线程已经不是同一线程。

实例:
@implementation TestClass    - (void) dealloc  {      // If you don‘t remove yourself as an observer, the Notification Center      // will continue to try and send notification objects to the deallocated      // object.      [[NSNotificationCenter defaultCenter] removeObserver:self];      [super dealloc];  }    - (id) init  {      self = [super init];      if (!self) return nil;        // Add this instance of TestClass as an observer of the TestNotification.      // We tell the notification center to inform us of "TestNotification"      // notifications using the receiveTestNotification: selector. By      // specifying object:nil, we tell the notification center that we are not      // interested in who posted the notification. If you provided an actual      // object rather than nil, the notification center will only notify you      // when the notification was posted by that particular object.        [[NSNotificationCenter defaultCenter] addObserver:self                                               selector:@selector(receiveTestNotification:)                                                    name:@"TestNotification"                                                 object:nil];        return self;  }    - (void) receiveTestNotification:(NSNotification *) notification  {      // [notification name] should always be @"TestNotification"      // unless you use this method for observation of other notifications      // as well.        if ([[notification name] isEqualToString:@"TestNotification"])          NSLog (@"Successfully received the test notification!");  }    @end

 

NSnotificationcenter